Servo actuator and position sensor

ABSTRACT

In an actuator having an integrated drive circuit, a switching current flowing in a coil within the actuator is included in a position sensor signal of the actuator. According to the present invention, by arranging a detection circuit which performs sampling in synchronization with noise, a rotational position which does not include higher harmonic waves can be detected even if the switching noise in a motor coil current in the actuator itself is generated. Consequently, elimination of the influence of the noise allows posture and position control with less vibration to be realized.

TECHNICAL FIELD

The present invention relates to servo actuators applicable tomulti-axis drive machinery, such as robots, general purpose assemblyequipment, robot hands, and other, types of multi-axis controllers. Inparticular, the present invention relates to a servo actuator and aposition detector therefor that can detect the posture and position of arotation axis with high accuracy.

More specifically, the present invention relates to a servo actuatorincluding a drive circuit and to a position detector for the servoactuator. In particular, the present invention relates to a servoactuator and a position detector therefor that detect the posture andposition of a rotation axis with high accuracy without being affected byswitching noise in a coil current from the drive circuit.

BACKGROUND ART

Machinery that performs in a manner similar to human behavior usingelectric or magnetic operations is called a “robot”. It is said that theterm “robot” derives from the Slavic word “ROBOTA (slave machine)”. InJapan, robots have become widely used since the late 1960′s. Many ofthem are industrial robots, such as manipulators and transfer robots,designed for the purpose of automation of production operations andunmanned production operations in factories.

Stationary robots, such as arm robots, which are installed and used inparticular places, operate only in a fixed and local workspace forassembly and selection of parts. In contrast, the workspace of mobilerobots is not limited. Mobile robots move along a predetermined path ormove freely. Therefore, mobile robots can perform predetermined or anyhuman operations in place of human beings and can offer various servicesreplacing human beings, dogs, and other living things. Among mobilerobots, although legged mobile robots are less stable and have moredifficulty in posture control and walking control compared to crawlerrobots and robots with tires, legged mobile robots are better in thatthey can climb up and down stairs and over obstacles, and can flexiblywalk and run regardless whether the ground is prepared or unprepared.

Recently, research and development on legged mobile robots, such as petrobots which emulate the physical mechanisms and operations ofquadrupedal walking animals such as dogs and cats, and “humanoid” or“human-shaped” robots (humanoid robots) designed on the model of thephysical mechanisms and operations of bipedal upright walking animalssuch as human beings, has advanced. There are increasing expectationsfor the practical use of legged mobile robots. For example, SonyCorporation released a bipedal walking humanoid robot “SDR-3X” on Nov.21, 2000.

Generally, a legged mobile robot of this type is equipped with manyjoint degrees-of-freedom, and movements of joints can be realized byactuator motors. Also, servo control is performed by calculating therotational position and speed of each motor, thus reproducing desirableoperation patterns and performing posture control.

In general, servo motors are used for realizing joint degrees-of-freedomfor robots because servo motors are easy to handle, provide high torquewith a compact body, and have excellent response. In particular, ACservo motors have no brush and are maintenance free, so that they can beapplied to automated machinery that is desired to operate in unmannedworkspaces, such as joint actuators for legged robots that can walkfreely. Each AC servo motor has a permanent magnet at a rotor and coilsat a stator, so that sinusoidal magnetic flux distribution andsinusoidal current cause running torque to the rotor.

Each legged mobile robot generally includes many joints. A servo motorwhich provides joint degrees-of-freedom is thus required to be designedand produced so as to be compact and highly efficient. For example, thespecification of Japanese Patent Application No. 11-33386 (JapaneseUnexamined Patent Application Publication No. 2000-299970), which hasbeen already assigned to the applicant of the present invention,discloses a compact AC servo actuator applicable to a joint actuator oflegged mobile robots. The AC servo actuator is directly connected to agear and is of the type in which a servo control system integrated intoone chip is housed in a motor unit.

For multi-axis machinery such as a legged mobile robot, rotationalposition of each axis must be stably detected with high accuracy inorder to correctly operate the machinery by a positioning command. Forexample, it is required for a legged mobile robot of the bipedal uprightwalking-type, such as a humanoid robot, to autonomously confirm its ownposture and position immediately after the power supply is switched onand to move each axis into a stable posture and position.

For this reason, the AC servo actuator which provides the rotationaldegrees of freedom of each joint must be provided with ahigher-precision rotational position detector in order to realize suchstabilization of the posture and position.

However, in the servo actuator for legged mobile robots described above,since an actuator unit includes an integrated drive circuit, there is aproblem in that a position sensor signal is affected by noise generatedin the drive circuit. More specifically, in the actuator unit,application of a magnetic field to a rotor causes the coil currentflowing in a stator coil to be switched on and off. Switching noise insuch a coil current is inevitably included in the position sensorsignal.

For example, an error in the measurement accuracy of a rotation axis dueto noise may result in the legged mobile robot becoming destabilized orit may even fall. If the body falls, this not only damages the robotitself but also causes accidents, such as injury to workers near thebody and destruction of the object it collided with.

A conventional arrangement of the position sensor is inadequate fordetecting the position of a rotation axis with high accuracy. Therefore,a sensor and a detection circuit in which noise influence is consideredare needed in order to structurally separate the effects of noise.

However, a position detector in which the influence of noise is reducedneeds a complicated circuit structure and high mechanical accuracy.Therefore, the detection sensor and the detection circuit become largerand expensive. Consequently, even the joint actuator itself becomeslarge and expensive, so that design and assembly of the overall robotbecomes difficult and the cost of the apparatus is thus increased.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an excellent compactservo actuator including a drive circuit and to provide a positiondetector for the servo actuator.

It is another object of the present invention to provide an excellentservo actuator and a position detector therefor that detect the postureand position of a rotation axis with high accuracy without beingaffected by switching noise in a coil current from the drive circuitincluded in the servo actuator.

In view of the above objects, according to a first aspect of the presentinvention, a servo actuator of a type having a permanent magnet at arotor and a coil at a stator for generating torque by a magnetic fluxdistribution and a current passing through the coil includes:

a casing that accommodates the rotor and the stator and supports therotor rotatably around a predetermined rotation axis;

a drive control unit for controlling the rotation of the rotor byperforming PWM (pulse width modulation) switching control on the currentpassing through the stator coil with a predetermined period;

a rotational position detection unit for detecting the rotationalposition of the rotor; and

a sampling control unit for sampling an output of the rotationalposition detection unit in synchronization with a switching period ofthe current passing through the stator coil, in the drive control unit.

Also, according to a second aspect of the present invention, a positiondetector for a servo actuator of a type having a permanent magnet at arotor and a coil at a stator for generating torque by a magnetic fluxdistribution and a current passing through the coil includes:

a drive control unit for controlling the rotation of the rotor byperforming PWM (pulse width modulation) switching control on the currentpassing through the stator coil with a predetermined period;

a rotational position detection unit for detecting the rotationalposition of the rotor; and

a sampling control unit for sampling an output of the rotationalposition detection unit in synchronization with a switching period ofthe current passing through the stator coil, in the drive control unit.

The present invention can be applied to a servo actuator having anintegrated drive control circuit. The servo actuator can be applied, forexample, to a joint actuator of a legged mobile robot. In this type ofservo actuator, the drive control unit and the rotational positiondetection unit are accommodated in the casing. Therefore, the rotationalposition detection unit is disposed close enough to the drive controlunit that the rotational position detection unit is affected byswitching noise from the drive control unit.

The rotational position detection unit can be arranged with acombination of a rotor sensor magnet installed on one end face of therotor approximately coaxially with respect to the rotation axis, thesurface of the rotor sensor magnet being sinusoidally magnetized, andtwo rotational position sensors with a phase difference of approximate90 degrees between each other arranged around the rotation axis atportions facing the rotor sensor magnet, the rotational position sensorsdetecting the magnetic flux density.

In the actuator having the integrated drive circuit, a switching currentflowing in each coil within the actuator is inevitably included in aposition sensor signal of the actuator. In other words, since therotational position detection unit is disposed near the drive controlunit, which functions as a switching noise source, the output signal ofeach rotational position sensor is superimposed with the switchingcurrent, which acts as noise.

Noise from the drive control unit includes this noise, whose fundamentalcomponent is shown by a waveform of the switching current, and othernoise generated by circuit resonance due to the current variation. Inthis case, the noise which is superimposed on the output signal can betreated as a periodic signal which is approximately synchronized withthe PWM switching period.

In the present invention, the output of the rotational positiondetection unit is sampled in synchronization with the switching periodof the current passing through the stator coil, in the drive controlunit. Accordingly, since the size of the signal whose period isidentical to that of sampling becomes zero, the influence of theswitching noise from the transistor can be eliminated from the sensorsignal on which the noise is superimposed, even if the current switchingnoise is superimposed on the sensor outputs of each rotational positionsensor.

A period in which the PWM switch is turned on corresponds to atransitional period of the coil current. During this period, theswitching current always fluctuates, thus causing the noise tofluctuate. It is therefore relatively difficult for the noisesuperimposed on the sensor to be eliminated.

In contrast, during the period in which the PWM switch is turned off,the current variation is relatively stable, even in the transitionalperiod of the coil current. The sampling control unit samples the outputof the rotational position detection unit in synchronization with theperiod in which the drive control unit turns off the current passingthrough the stator coil, or in synchronization with a timing immediatelybefore the drive control unit turns on the current passing through thestator coil. Consequently, the influence of the switching noise can bereduced.

In particular, in a dead band region allocated immediately before thePWM switch is turned on, it is ensured that the PWM switch is turnedoff, thus providing a stable period with minimum noise. In other words,in the dead band, the noise component contained in the sensor output isapproximately constant and small.

Consequently, the sampling control unit can most efficiently reduce theinfluence of the switching noise for sampling the output of therotational position detection unit in synchronization with the deadband.

Further objects, features, and advantages of the present invention willbecome more apparent from the following more detailed description withreference to an embodiment of the present invention and the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a sectional structure of a servo motor1 according to an embodiment of the present invention taken along theaxial direction.

FIG. 2 is an illustration schematically showing that the surface of arotor sensor magnet 15 is sinusoidally magnetized.

FIG. 3 is an illustration schematically showing that two rotationalposition sensors 16A and 16B with a phase difference of 90 degrees withrespect to a rotation axis are arranged on the surface near a rotor 11,of a control circuit board 13.

FIG. 4 is an illustration showing an example of the configuration of afeedback tracking circuit for obtaining a rotational position θ_(m) fromtwo sensor signals SIN (θ_(m)) and COS (θ_(m)).

FIG. 5 is an illustration showing the configuration of an approximatelyequivalent circuit of the feedback tracking circuit shown in FIG. 4.

FIG. 6 is an illustration showing an example of the configuration of adigital circuit by which the rotational position θ_(m) of the rotor 11can be obtained from the sensor signals SIN (θ_(m)) and COS (θ_(m)) ofhall sensors 16A and 16B.

FIG. 7 is a timing chart showing the relationship between input examplesof analog sensor signals SIN (θ_(m)) and COS (θ_(m)) of the rotorsensors 16A and 16B and an output θ_(xd) from the digital circuit shownin FIG. 6 in this case.

FIG. 8 is an illustration showing an example of the configuration of anequivalent circuit of a current control circuit 20, which is applied toa servo actuator 10 according to the present embodiment, for supplying acoil current.

FIG. 9 is a timing chart showing the relationship between PWM switchingof each transistor and switching current in the current control circuit20, and more particularly, showing a voltage waveform of a coilterminal.

FIG. 10 is a timing chart showing the relationship between the PWMswitching of each transistor and the switching current in the currentcontrol circuit 20, and more particularly, showing the waveform of thecoil current.

FIG. 11 is a timing chart showing the relationship between the PWMswitching of each transistor and the switching current in the currentcontrol circuit 20.

FIG. 12 is a timing chart showing that the sensor output signals SIN(θ_(m)) and COS (θ_(m)) of the rotational position sensors 16A and 16Bhave noise superimposed thereon.

FIG. 13 is a timing chart showing that the sensor output signals SIN(θ_(m)) and COS (θ_(m)) are sampled in synchronization with a PWMswitching period T_(PWM) from the rotational position sensors 16A and16B on which switching noise is superimposed.

FIG. 14 is an illustration showing an equivalent circuit of a feedbacktracking circuit that can treat noise N_(d) which is superimposed on thesensor output as a constant value by sampling the outputs of therotational position sensors 16A and 16B (the tracking circuit is mountedin a drive circuit 13A).

FIG. 15 is a block diagram showing a servo control configuration of theservo actuator according to the present embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described in detailwith reference to the drawings.

FIG. 1 shows a sectional structure of a servo actuator 10 according toan embodiment of the present invention in the axial direction.

As shown in the drawing, the servo actuator 10 includes, for example, athree-phase stator 12 that is arranged in the circumferential directionaround a rotor 11 having a predetermined rotation axis. The rotor 11 issupported so as to be rotatable around the predetermined rotation axis.These rotor 11 and stator 12 are accommodated in a substantiallycylindrical casing and constitute a single servo actuator unit.

A permanent magnet is disposed at the rotor 11 and coils are disposed atthe stator 12. A sinusoidal current is supplied to the coils to form apredetermined sinusoidal magnetic flux distribution, thus allowingrunning torque to be applied to the rotor 11.

In the present embodiment, the stator 12 includes three-phase coilscomposed of U, V, and W phases (described below). Applying analternating current to each phase generates a magnetic field, thusenabling torque to be applied to the rotor 11. Also, the control of thecurrent applied to each phase enables the control of the running torqueapplied to the rotor 11.

In order to achieve a servo motor which is smaller in size and whichprovides higher output, a magnet with a high magnetic flux density maybe used at the rotor. For example, a polar anisotropic magnet has a highmagnetic flux density, and thus is excellent in achieving higher output.For example, the specification of Japanese Patent Application No.2000-128409 (Japanese Unexamined Patent Application Publication No.2001-314050), which has been already assigned to the applicant of thepresent invention, discloses a servo actuator using a polar anisotropicmagnet at the rotor in order to reduce the size and increase the output.

Also, in order to achieve a servo motor which is smaller in size andwhich provides higher output, each coil of the stator is made moredense. For example, a split core type is adopted for the stator. For thesplit core type, iron cores, or cores, are arranged in thecircumferential direction. Further, after the coils are neatly wound ina separate step, each of them is set up, thus forming the stator.Accordingly, high-density core winding can be obtained, thus making itpossible to save space for the actuator. For example, the specificationof Japanese Patent Application No. 2000-281072 (Japanese UnexaminedPatent Application Publication No. 2002-95192), which has been alreadyassigned to the applicant of the present invention, discloses a servoactuator which adopts a stator of a split core type in order to suitablywind coils.

The servo actuator 10 according to the present embodiment is a compactactuator including a drive circuit 13A accommodated in the same casing.In the example shown in FIG. 1, printed wiring of a predeterminedpattern is laid on a control circuit board 13. Also, the drive circuit13A and its neighboring circuit chips are mounted on the control circuitboard 13. The control circuit board 13 is arranged in an approximatelydisk shape. In the approximate center of the control circuit board 13,an opening for inserting a rotating shaft of the rotor 11 is provided.

A ring rotor sensor magnet 15 is installed on one end face near thecontrol circuit board 13, of the rotor 11. The surface of the ring rotorsensor magnet 15 is sinusoidally magnetized, as shown in FIG. 2.

The surface of the ring rotor sensor magnet 15 is sinusoidallymagnetized (described above), and its magnetic flux density φ isexpressed as a function of the rotational position θ_(m) of the rotor11. In the present embodiment, the magnetic flux density φ(θ_(m)) of therotor sensor magnet 15 is expressed by the following equation. Thepositions and number of poles of the sensor magnet 15 are identical tothose of a rotating magnet 14.

φ(θ_(m))=φ₀×sin (θ_(m))

The rotational position θ_(m) of the rotor 11 arbitrarily changes. Here,the equation θ_(m)=0 is defined as expressing a home position of amagnetic polar axis of an output magnet for the rotor 11.

In contrast, two rotational position sensors 16A and 16B with a phasedifference of 90 degrees are arranged with reference to the rotationaxis on the surface near the rotor 11, of the control circuit board 13,as shown in FIG. 3.

The rotational position sensors 16A and 16B include elements (hallelements) which detect the magnetic flux density in the home position ofthe magnetic polar axis. The rotational position sensor 16A outputs ahall sensor signal SIN corresponding to a magnetic field generated bythe rotational sensor magnet. The rotational position sensor 16Bsimilarly outputs a hall sensor signal COS. These SIN signal and COSsignal functioning as sensor outputs are input to the drive circuit 13A.

These hall sensor signals SIN and COS represent the rotational positionof the servo actuator 10. In other words, the drive circuit 13A performsfeedback control of the rotational drive of the actuator motor on thebasis of a positioning command from the outside (for example, a maincontroller). For example, in the case that the servo actuator 10 isapplied to a joint actuator of a legged mobile robot, the feedbackcontrol of the actuator motor is strongly related to the posture andstability control of the body. FIG. 15 shows a servo configuration ofthe servo actuator 10.

Each of the hall sensor signals SIN and COS is a function of therotational position θ_(m) of the rotor 11. In the present embodiment,they are expressed as follows: $\begin{matrix}{{{SIN}\left( \varphi_{m} \right)} = {{G_{0}(t)} \times {\varphi_{0}(t)} \times \sin \quad \varphi_{m}}} \\{{{COS}\left( \varphi_{m} \right)} = {{{G_{0}(t)} \times {\varphi_{0}(t)} \times \left( {{\sin \quad \varphi_{m}} + \frac{1}{2\pi}} \right)} = {{G_{0}(t)} \times {\varphi_{0}(t)} \times \cos \quad \theta_{m}}}}\end{matrix}$

Here, G₀(t) represents a sensitivity coefficient of the hall sensors.Absolute temperature is represented by t, and φ₀(t) and G₀(t) varydepending on t. Also, φ₀(t)G₀(t) is a positive constant The rotationalposition θ_(m) can be obtained from these two sensor signals SIN (θ_(m))and COS (θ_(m)) by a feedback tracking circuit FIG. 4 shows an exampleof the configuration of the feedback tracking circuit.

The error function E(θ) of the feedback tracking system is expressed asfollows:

−E(θ)=SIN (θ _(m))×COS (θ_(X))−COS (θ_(m))×SIN (θ_(X))=SIN (θ_(m)−θ_(X))

If the condition that the error function E(θ) is stable, or converges tozero, is satisfied, the following equation holds. Accordingly, θ_(m) isobtained.

θ_(X)=θ_(m)

In this case, the feedback tracking circuit shown in FIG. 4 can beregarded as the configuration of the approximately equivalent circuitshown in FIG. 5. The value of the pole P in the system shown in thedrawing is expressed as follows:$P = \frac{{- K_{p}} \pm \sqrt{K_{p}^{2} - {4K_{p}K_{i}}}}{2}$

Consequently, for the stability condition for this system, coefficientsK_(p) and K_(i) are set so that the value of the pole P is negative.Therefore, the stability condition P<0 becomes K_(p)>0 and K_(i)>0. Theresponse frequency is obtained by the square root of the product ofthese coefficients K_(p) and K_(i).

By the value of θ_(x), which is obtained as described above, theposition can be detected independently of the sizes of the coefficientsG₀(t) and φ₀(t), which are dependent on temperature t. Consequently,variations in the measurement accuracy of the rotational position due totemperature can be suppressed.

The operation to obtain the rotational position θ_(m) of the rotor 11from the sensor signals SIN (θ_(m)) and COS (θ_(m)) from the hallsensors 16A and 16B can be achieved by any mode of implementation, suchas use of a special hardware circuit or execution of a software program.

FIG. 6 illustrates an example of the configuration of a digital circuitby which the rotational position θ_(m) of the rotor 11 can be obtainedfrom the sensor signals SIN (θ_(m)) and COS (θ_(m)) from the hallsensors 16A and 16B. In the drawing, if an overflow value is entered,clips 1 and 2 fix the upper or lower limits at the maximum or minimumvalues of data 32 bits in length. The value of G₁ may be any figure aslong as it is positive.

The analog sensor signals SIN (θ_(m)) and COS (θ_(m)) are held and inputby a zero-order hold circuit ZOH during a sampling time with a certainperiod. The equation of the zero-order hold circuit ZOH(s) is thefollowing equation:${{ZOH}(s)} = \frac{1 - {\exp \left( {{- s} \times T_{0}} \right)}}{s}$

In the above equation, s represents the Laplace operator, and T₀represents the sampling period. Also, s can be expressed as jw. By usingthe equation ωs=2π/T₀, the above equation can be transformed into thefollowing equation: $\begin{matrix}{{{ZOH}({jw})} = \frac{1 - {\exp \left( {{- {jw}} \times T_{0}} \right)}}{jw}} \\{= {\frac{2{\pi \cdot {{SIN}\left( {\pi \cdot \frac{\omega}{\omega \quad s}} \right)}}}{\omega \quad {s \cdot \frac{\pi\omega}{\omega \quad s}}} \cdot {\exp \left( {{- {j\pi}} \cdot \frac{\omega}{\omega \quad s}} \right)}}}\end{matrix}$

In other words, it can be seen from the above equation that, if an inputsignal expressed by the equation ω=ωs is input, the signal frequency ωbecomes ZOH(jw)=0.

It is also known from the above equation that the original signal can bereproduced if the frequency of the signal is sufficiently lower than thesampling period.

In the circuit configuration shown in FIG. 6, the frequency of thesensor signals SIN (θ_(m)) and COS (θ_(m)) is sufficiently lower thanthe sampling period. In the present embodiment, the sampling frequencyis set to 20 KHz (50 μs), and the maximum frequency of the sensorsignals SIN (θ_(m)) and COS (θ_(m)) is set to 1 KHz.

Assuming that analog sensor signals SIN (θ_(m)) and COS (θ_(m)) shown inFIG. 7 are input to the rotor sensors 16A and 16B, an output θ_(xd) ofan operation result from the digital circuit at this time becomes asshown in the drawing. However, in practice, the output θ_(xd) from thedigital circuit is a discrete integer value.

The operation by the digital circuit shown in FIG. 6 is arranged so thatevery operation is performed within a predetermined sampling period. Inother words, the circuit output θ_(xd) is updated every sampling period(50 μs)

As described above, in the servo actuator 10, applying the current tothe motor coils wound around the stator 12 generates a magnetic field,thus generating running torque on the rotor 11 which is composed of amagnet. More specifically, the sinusoidal current is applied to themotor coils in order to form a sinusoidal magnetic flux distribution.Then, the servo control of the coil current is performed on the basis ofthe sensor output including rotational position and speed.

In general, the coil current applied to the coils of each of theU-phase, V-phase, and W-phase, which constitute the stator 12, iscontrolled by a current control circuit which includes aswitching-operating transistor element.

FIG. 8 illustrates an example of the configuration of an equivalentcircuit of a current control circuit 20, which is applied to the servoactuator 10 according to the present embodiment, for supplying a coilcurrent. Such a current control circuit 20 is, for example, arranged atevery coil of each phase included in the stator 12.

The current control circuit 20 has a full-bridge configuration includinga circuit in which two transistors Q1 and Q2 are connected to each otherin the forward direction and another circuit in which two transistors Q3and Q4 are similarly connected to each other in the forward direction.These two circuits are connected in parallel between a power supplyvoltage V_(cc) and a ground GND. Also, the midpoint of the transistorsQ1 and Q2 and the midpoint of the transistors Q3 and Q4 are connected toeach other using a single-phase coil Z1 of the stator 12. Here, Z1represents one of the U-phase, the V-phase, and the W-phase. Phasesother than these phases are arranged by a circuit similar to the circuitshown in the drawing.

Switching on the transistors Q1 and Q4 and switching off the transistorsQ2 and Q3 causes a current A to flow in the coil Z1 in the directionindicated by the arrow in the drawing. Then, switching off thetransistors Q2 and Q3 and switching off the transistors Q1 and Q4 causescurrents A and B to flow in the coil Z1.

The period in which the current A flows by switching on the transistorsQ1 and Q4 and switching off the transistors Q2 and Q3 is defined as aregion A. The period in which the current B flows by switching off thetransistors Q2 and Q3 and switching off the transistors Q1 and Q4 isdefined as a region B.

The current I₁ flowing in the coil Z1 is a switching current which isdetermined by the switching control of each transistor. The size of theswitching current I₁ is determined by a PWM (pulse width modulation)switch, or time widths of the regions A and B.

FIGS. 9 and 10 show the relationship between the PWM switching of eachtransistor and the switching current in the current control circuit 20.(FIG. 9 shows a voltage waveform of a coil terminal, and FIG. 10 shows acoil current waveform.) T_(on) represents a pulse width which isdetermined by the length of the region A, and T_(PWM) represents acertain period of the PWM switching. For example, if T_(on) is 30 μs andT_(PWM) is 50 μs, the current I₁ which flows in the coil becomes asshown in FIG. 10.

In general, a PWM switching signal is arranged to control the size ofthe coil current I₁. Its maximum current is determined by the maximumpulse width. The maximum pulse width T_(onA) is determined by themaximum period of the transitional period which is required forswitching on and off of each transistor which constitutes the currentcontrol circuit 20. In other words, in consideration of the transitionalperiod required for switching on and off of each transistor, the upperlimit T_(onA) of the pulse width is set in order that one pair oftransistors Q1 and Q4 and the other pair of transistors Q2 and Q3 arenot switched on at the same time.

The remainder obtained by subtracting the maximum pulse width T_(onA)from the PWM switching period T_(PWM) ensures a dead band. FIG. 11schematically illustrates that the dead band is ensured by the PWMswitching control by the current control circuit 20. In the exampleshown in the figure, the PWM switching period T_(PWM) is 50 μs, and adead band of 1 μs is ensured. Consequently, the maximum pulse widthT_(onA) becomes 49 μs.

A signal containing a higher harmonic wave of the switching current insuch a current control circuit is defined as a basic noise waveformgenerated by the drive circuit 13A. The frequency of this waveform canbe regarded as a periodic signal which is dependent on the switchingfrequency.

In the servo actuator 10 according to the present embodiment, as shownin FIG. 1, the circuit board for servo control, such as the drivecircuit 13A, is included in the actuator unit which accommodates theactuator main body, such as the rotor 11 and the stator 12, and isarranged integrally in the actuator unit. In other words, the,rotational position sensors 16A and 16B for detecting the rotationalposition of the rotor 11 are disposed near the drive circuit 13A, whichfunctions as a noise source. Consequently, switching current in thecurrent control circuit 20 is inevitably superimposed on the outputsignals SIN (θ_(m)) and COS (θ_(m)) of the rotational position sensors16A and 16B as noise.

More specifically, noise includes this noise, whose fundamentalcomponent is shown by a waveform of the switching current, and othernoise generated by circuit resonance due to the current variation. Inthis case, the noise which is superimposed on the output signals SIN(θ_(m)) and COS (θ_(m)) can be regarded as a periodic signal which issynchronized with the PWM switching period T_(PWM). FIG. 12 shows thatthe sensor output signals SIN (θ_(m)) and COS (θ_(m)) of the rotationalposition sensors 16A and 16B have noise superimposed thereon.

For example, noise elimination can be achieved by a structure in whichthe current control circuit and the rotational position sensors aremechanically separated from each other and a circuit in which thecurrent control circuit and the rotational position sensors areelectrically isolated and separated from each other. However, providingsuch a mechanical and electrical design for the servo actuator causesthe apparatus configuration to become large. This results in preventingminiaturization of the actuator and increasing the apparatus cost.

In the present invention, the rotational position sensors 16A and 16Bare arranged so that sampling is performed in synchronization with thePWM signal of the current control circuit, which is the fundamentalfrequency of the current noise. With this arrangement, since the size ofthe signal whose period is identical to that of sampling becomes zero,even if the current switching noise is superimposed on the sensoroutputs SIN (θ_(m)) and COS (θ_(m)) of the rotational position sensors16A and 16B, the influence of the switching noise from the transistorcan be eliminated from the sensor signal on which noise is superimposed.

Referring to FIG. 11, the region A corresponding to the period in whichthe PWM switch is turned on corresponds to the transitional period ofthe coil current. During this period, the switching current alwaysfluctuates, thus causing the noise to fluctuate. It is thereforerelatively difficult for the noise superimposed on the sensor to beeliminated.

In contrast, in the region B corresponding to the period in which thePWM switch is turned off, current variation is relatively stable even inthe transitional period of the coil current. In particular, in the deadband region allocated immediately before the PWM switch is turned on, itis ensured that all transistors constituting the PWM switch are turnedoff, thus-providing a stable period with minimum noise. In other words,in the dead band, the noise component contained in the sensor outputsSIN (θ_(m)) and COS (θ_(m)) is approximately constant and small.

Consequently, by arranging the rotational position sensors 16A and 16Bso as to be sampled in synchronization with the dead band region, theinfluence of switching noise can be most efficiently reduced.

Among the noise frequency components contained in a true signal,frequencies higher than the response frequency of the rotationalposition detection system are attenuated, thus enabling the influence ofthe noise components having frequency more than the required frequencyto be attenuated. In the frequency range handled by the rotationalposition detection system, since such noise is constant as describedabove, the signal can also be kept constant. In other words, the erroris constant. Therefore, the output accuracy includes a constant error.

In general, servo motors often handle speed signals as well as positionsignals. In that case, speed may be detected using the difference ofposition and using a state observer. In any case, a differentialposition signal is used. It is known that constant noise does notinfluence the detection system.

In the state in which a current is not applied to the motor and the highfrequency noise is small (in other words, in the region B andimmediately before the region A) in the rotational position detectionsystem according to the present embodiment, a current is not applied tothe stator coil. Therefore, more accurate position information can beobtained. Even if large a current flows in the stator coil and the ratioof noise thus becomes increased, the ratio is kept constant.Accordingly, high-frequency noise which causes adverse effects on themeasurement accuracy of the rotational position can be reduced.

FIG. 12 shows waveforms of the sensor output signals SIN (θ_(m)) and COS(θ_(m)) of the rotational position sensors 16A and 16B on which noise issuperimposed. The sensor signals are input in synchronization with thePWM switching period T_(PWM) indicated by dotted lines, so that theinfluence of errors due to switching noise can be reduced. Inparticular, the influence at high frequencies is significantlyattenuated. Although the influence of noise appears in the sensor outputsignals SIN (θ_(m)) and COS (θ_(m)) as a mode of offset, the offset hasa small effect on the detection system.

FIG. 13 shows that the sensor output signals SIN (θ_(m)) and COS(θ_(m))are sampled in synchronization with the PWM switching period T_(PWM)from the rotational position sensors 16A and 16B on which switchingnoise is superimposed. An example shown in the figure shows that thesensor output signals SIN (θ_(m)) and COS (θ_(m)) are sampled insynchronization with the PWM switching period T_(PWM) using the deadband region.

The sensor signals containing noise generated by the switching currentin the current control circuit are produced by superimposingapproximately constant-sized noise, which is similar to the currentwaveform, on the true sensor signals SIN (θ_(m)) and COS (θ_(m)) Thewaveforms of the sensor signals are expressed by the full lines shown inFIG. 13.

During this time, if the signals immediately before t₁, that is aftert_(a), are sampled in synchronization with the switching frequency ofthe current control circuit with a predetermined period T_(PWM), signalscan be obtained at each point C₀, C₁, S₀, and S₁, as shown in FIG. 13.

These signals at points C₀, C₁, S₀, and S₁ are constant values due tothe characteristics of sampling. Assuming that the difference of thethus obtained signals and the true sensor signals SIN (θ_(m)) and COS(θ_(m)) is N_(d), then N_(d) can be expressed as follows:

N _(d) ≈C ₀−COS (θ_(m))≈S ₀−SIN (θ_(m))≈constant

In the above equation, since the sampling interval T_(PWM) issufficiently smaller than the time variation of θ_(m), the followingequations can be obtained: C₀=C₁ and S₀=S₁.

As described above, by sampling the outputs of the rotational positionsensors 16A and 16B in synchronization with the signals due to switchingnoise, the noise N_(d) which is superimposed on the sensor outputs canbe regarded as a constant value. Therefore, the feedback trackingcircuit for obtaining the rotational position θ_(x) from the two sensorsignals SIN (θ_(m)) and COS (θ_(m)) can be expressed by an equivalentcircuit shown in FIG. 14.

In the configuration of the equivalent circuit shown in FIG. 14, theerror function E(θ) can be expressed as follows: $\begin{matrix}{{E(\theta)} = {{\left( {{{SIN}\left( \theta_{m} \right)} + N_{d}} \right) \times {{COS}\left( \theta_{X} \right)}} - {\left( {{{COS}\left( \theta_{m} \right)} + N_{d}} \right) \times {{SIN}\left( \theta_{X} \right)}}}} \\{= {{{SIN}\left( {\theta_{m} - \theta_{x}} \right)} + {N_{d}\left( {{{COS}\left( \theta_{x} \right)} - {{SIN}\left( \theta_{x} \right)}} \right)}}}\end{matrix}$

The noise size N_(d) of the signal, which is sampled in synchronizationwith the noise, can be made smaller and is constant. Therefore, thestate of convergence is expressed as E(θ)=0 from the convergencecondition of the detection system. Accordingly, the output θ_(x) of theequivalent circuit shown in FIG. 14 can be expressed by an approximateequation as follows:

θ_(x)≈θ_(m) +N _(d)( COS (θ_(x))−SIN (θ_(x))

As described above, the noise N_(d) is regarded as a constant value.Therefore, θ_(x) is a signal of a true rotational position θ_(m)containing a constant offset in which a higher harmonic waveaccompanying transistor switching of the current control circuit is notincluded. Also, as is clear from the above equation, if the equationsθ_(x)=π/4 and θ_(x)=3π/4 are satisfied, there is a measurement positionat which the influence of noise becomes zero.

Accordingly, because the noise N_(d) is regarded as being constant, theoutput θ_(x) of the equivalent circuit is shown as a continuous curvewhich varies depending on the noise, as shown in FIG. 11.

FIG. 15 shows a servo control configuration of the servo actuatoraccording to the present embodiment.

The feedback system shown in FIG. 15 handles state variables of aposition signal and its differential signal. If noise containing somehigher harmonic waves is included in the position signal θ_(x) orθ_(xd), a problem occurs in that vibration is excited by noise in thissystem.

In accordance with the present invention, even if the servo actuator isarranged to be compact and includes the integrated drive circuit byarranging the rotational position sensors and the current controlcircuit close to each other, the influence of the switching noise can bereduced, as described above.

APPENDIX

While the present invention has been described in detail with referenceto a particular embodiment, it is obvious that modifications andsubstitutions of the embodiment may be made by those skilled in the artwithout departing from the scope of the present invention. In otherwords, the present invention has been disclosed as merely an example andshould not be interpreted in a restrictive manner. In order to determinethe scope of the present invention, the claims section at the beginningof the Description should be considered.

INDUSTRIAL APPLICABILITY

According to the present invention, an excellent servo actuator andposition detector therefor which are applicable to multi-axis drivemachinery, such as robots, general purpose assembly equipment, robothands, and other types of multi-axis controllers can be provided.

Also, according to the present invention, an excellent servo actuatorincluding a drive circuit and a position detector for the servo actuatorcan be provided.

Further, according to the present invention, an excellent servo actuatorand a position detector therefor that can detect the posture andposition of a rotation axis with high accuracy without being affected byswitching noise in a coil current from the drive circuit included in theservo actuator can be provided.

In the servo actuator according to the present invention, arranging of adetection circuit for performing sampling in synchronization with noiseenables a rotational position which does not contain higher harmonicwaves to be detected, even if switching noise in a motor coil current ofthe actuator itself is generated. Also, the position detector accordingto the present invention can be arranged so as to be compact andinexpensive, thus not preventing miniaturization of the servo actuatorand not increasing the cost of the overall apparatus.

In the case that the servo actuator according to the present inventionis applied to a bipedal upright-walking legged mobile robot, theinfluence of noise can be eliminated, thus allowing posture and positioncontrol with less vibration to be realized. Also, since the size of theservo, actuator is reduced, the size of a portion near each joint doesnot increase. Therefore, a balanced and attractive body can be designed.

In general, in the feedback control system of the servo actuator,differentiation of a position signal, is often used. Therefore, a signalcontaining higher harmonic noise waves causes more adverse effects onthe system than the problem of noise offset. According to the presentinvention, by sampling sensor signals at a timing that is synchronizedwith transistor switching in a current control circuit, such a problemof the higher harmonic waves can be eliminated. Consequently, in theservo actuator having the integrated drive circuit, more stable controlcan be achieved by using a small and inexpensive circuit.

What is claimed is:
 1. A servo actuator of a type having a permanentmagnet at a rotor and a coil at a stator for generating torque by amagnetic flux distribution and a current passing through the coil, theservo actuator comprising: a casing that accommodates the rotor and thestator and supports the rotor rotatably around a predetermined rotationaxis; a drive control unit for controlling the rotation of the rotor byperforming PWM (pulse width modulation) switching control on the currentpassing through the stator coil with a predetermined period; arotational position detection unit for detecting the rotational positionof the rotor; and a sampling control unit for sampling an output of therotational position detection unit in synchronization with a switchingperiod of the current passing through the stator coil, in the drivecontrol unit.
 2. A servo actuator according to claim 1, wherein thedrive control unit and the rotational position detection unit areaccommodated in the casing, and wherein the rotational positiondetection unit is disposed close enough to the drive control unit thatthe rotational position detection unit is affected by switching noisefrom the drive control unit.
 3. A servo actuator according to claim 1,wherein the rotational position detection unit includes: a rotor sensormagnet installed on one end face of the rotor approximately coaxiallywith respect to the rotation axis, wherein the surface of the rotorsensor magnet is sinusoidally magnetized; and two rotational positionsensors with a phase difference of approximate 90 degrees between eachother arranged around the rotation axis at portions facing the rotorsensor magnet, the rotational position sensors detecting the magneticflux density.
 4. A servo actuator according to claim 1, wherein thesampling control unit samples the output of the rotational positiondetection unit in synchronization with a period in which the drivecontrol unit turns off the current passing through the stator coil.
 5. Aservo actuator according to claim 1, wherein the sampling control unitsamples the output of the rotational position detection unit insynchronization with a timing immediately before the drive control unitturns on the current passing through the stator coil.
 6. A servoactuator according to claim 1, wherein the drive control unit includes adead band which maintains an “off” state for a predetermined period oftime immediately before the current passing through the stator coil isturned on, and wherein the sampling control unit samples the output ofthe rotational position detection unit in synchronization with the deadband.
 7. A position detector for a servo actuator of a type having apermanent magnet at a rotor and a coil at a stator for generating torqueby a magnetic flux distribution and a current passing through the coil,the position detector comprising: a drive control unit for controllingthe rotation of the rotor by performing PWM (pulse width modulation)switching control on the current passing through the stator coil with apredetermined period; a rotational position detection unit for detectingthe rotational position of the rotor; and a sampling control unit forsampling an output of the rotational position detection unit insynchronization with a switching period of the current passing throughthe stator coil, in the drive control unit.
 8. A position detector for aservo actuator according to claim 7, wherein the drive control unit andthe rotational position detection unit are accommodated in a casingwhich accommodates the rotor and the stator and supports the rotorrotatably around a predetermined rotation axis, and wherein therotational position detection unit is disposed close enough to the drivecontrol unit that the rotational position detection unit is affected byswitching noise from the drive control unit.
 9. A position detector fora servo actuator according to claim 7, wherein the rotational positiondetection unit includes: a rotor sensor magnet installed on one end faceof the rotor approximately coaxially with respect to the rotation axis,wherein the surface of the rotor sensor magnet is sinusoidallymagnetized; and two rotational position sensors with a phase differenceof approximate 90 degrees between each other arranged around therotation axis at portions facing the rotor sensor magnet, the rotationalposition sensors detecting the magnetic flux density.
 10. A positiondetector for a servo actuator according to claim 7, wherein the samplingcontrol unit samples the output of the rotational position detectionunit in synchronization with a period in which the drive control unitturns off the current passing through the stator coil.
 11. A positiondetector for a servo actuator according to claim 7, wherein the samplingcontrol unit samples the output of the rotational position detectionunit in synchronization with a timing immediately before the drivecontrol unit turns on the current passing through the stator coil.
 12. Aposition detector for a servo actuator according to claim 7, wherein thedrive control unit includes a dead band which maintains an “off” statefor a predetermined period of time immediately before the currentpassing through the stator coil is turned on, and wherein the samplingcontrol unit samples the output of the rotational position detectionunit in synchronization with the dead band.